Genes encoding nicotine biosynthesis enzymes are known. For example, the tobacco quinolate phosphoribosyl transferase (QPT) gene has been cloned; see U.S. Pat. No. 6,423,520 and Sinclair et al., Plant Mol. Biol. 44: 603-17 (2000). QPT suppression provides significant nicotinic alkaloid reductions in transgenic tobacco plants. Xie et al., Recent Advances in Tobacco Science 30: 17-37 (2004). Likewise, suppression of an endogenous putrescine methyl transferase (PMT) sequence has been shown to reduce nicotine levels but increase anatabine levels by about 2-to-6-fold. Hibi et al., Plant Cell 6: 723-35 (1994); Chintapakorn and Hamill, Plant Mol. Biol. 53:87-105 (2003); Steppuhn et al. PLoS Biol 2:8:e217: 1074-1080 (2004). Levels of nicotine and other nicotinic alkaloids are reduced in tobacco by suppressing either the A622 or NBB1 nicotine biosynthesis genes. See WO/2006/109197.
Despite this, a comprehensive understanding of how the nicotine biosynthetic pathway functions is essential. Accordingly, further research efforts have been underway to elucidate the biochemistry and molecular biology of this pathway, including the identification of all related genes. Additional insights into biosynthesis pathways of other nicotinic alkaloids and of other alkaloids found in non-Nicotiana plants would be facilitated by a comprehensive understanding of the nicotine biosynthesis pathway in tobacco.
Reducing total alkaloid content in tobacco would increase the value of tobacco as a biomass resource. Reduced-alkaloid tobacco is more amenable for non-traditional purposes, such as biomass and derived products. For example, it is advantageous to use reduced-alkaloid tobacco for producing ethanol and protein co-products. Additionally, alkaloid-free tobacco or fractions thereof may be used as a forage crop, animal feed, or a human nutritive source. See WO/2002/098208.
An additional use of reduced-nicotine tobacco is for smoking cessation. Nicotine-reduced or nicotine-free tobacco cigarettes have assisted smokers in quitting smoking. Additionally, denicotinized cigarettes relieve craving and other smoking withdrawal symptoms. See Rose, Psychopharmacology 184: 274-285 (2006) and Rose et al., Nicotine Tobacco Res. 8: 89-101 (2006).
It may be beneficial to overexpress a nicotine biosynthesis gene, as means for increasing nicotine biosynthesis and accumulation in tobacco. For example, because nicotinic alkaloids play an important role in protecting plants against insects and herbivores, it is likely to be advantageous to increase nicotinic alkaloid synthesis in a host plant. From an herbivory perspective, increased nicotine synthesis and accumulation would provide an environmentally acceptable means for mediating plant-pest interactions.
As nicotine is the physically and psychologically active component in cigarette smoke, it may be advantageous to increase nicotine content in tobacco by genetic engineering. Research studies demonstrate that when supplementary nicotine is added to cigarette tobacco from an outside source, smokers inhale less of the more harmful components of smoke such as tar and carbon monoxide. See Armitage et al., Psychopharmacology 96: 447-53 (1988), Fagerström, Psychopharmacology 77: 164-67 (1982), Russell, Nicotine and Public Health 15: 265-84 (2000), and Woodman et al., European Journal of Respiratory Disease 70: 316-21 (1987). Likewise, a report by The Institute of Medicine of the U.S. on potential reduced exposure products (PREPS) concluded that “retaining nicotine at pleasurable or addictive levels while reducing the more toxic components of tobacco is another general strategy for harm reduction.” See CLEARING THE SMOKE ASSESSING THE SCIENCE BASE FOR TOBACCO HARM REDUCTION, IOM at page 29 (2001), commonly referred to as the “TOM Report” by the tobacco industry.
The part of the nicotine biosynthesis pathway that produces the N-methylpyrrolinium cation also is part of the pathway for the biosynthesis of other alkaloids, including medicinal tropane alkaloids. Hashirnoto and Yamada, Annu. Rev. Plant Physiol. Plant Mol. Biol. 45: 257-285 (1994); Kutchan, T. M., “Molecular genetics of plant alkaloid biosynthesis. In: Cordell, G. A. (ed.) 50 ALKALOIDS 257-316 (Academic Press, 1998).
A plant also can be genetically engineered to regulate its alkaloid profile, such as the ratio of a particular alkaloid to total alkaloid content. For example, if the goal is increasing the ratio of anatabine to total alkaloid content of a N. tabacum plant, PMT is suppressed. Chintapakorn and Hamill, supra.
As more alkaloid biosynthesis genes are discovered, including an understanding of their function and location in alkaloid biosynthesis pathways, the more sophisticated genetic engineering of these pathways can become. Accordingly, there is a continuing need to identify additional genes whose expression can be regulated to not only decrease or increase alkaloid(s) but to alter a plant's alkaloid profile, in particular, nicotinic alkaloids in N. tabacum plants.